The present invention relates to monitoring of inventory and to process measurement, and, more particularly, to a system and method for measuring the content of a bin.
The monitoring of liquid inventory generally is straightforward. By contrast, the monitoring of bulk solid inventory that consists of particulates piled up inside a to bin such as a silo often is very difficult. Examples of such bulk solid inventory include cement and sand for construction, grain, fertilizer, etc. The measurement of the level of bulk materials inside a bin is a problem that has not yet been solved adequately. The conditions inside bins typically are unfavorable (dust, extreme temperatures, etc.) and the contents of the bulk material stored in the bins often do not have a flat surface and are not always isotropic. Other difficulties arises from the wide variety of bin shapes in use and from the explosive atmospheres inside some bins.
The scope of the term “bin” as used herein includes any storage container, for bulk particulate solids, whose structure defines an interior volume for receiving and storing the solids. Such a bin may be closed above, below and on all sides, as is the case when the bin is a silo, vessel or tank, or may be open above or on one or more sides. The example of a “bin” that is used in the detailed description of the present invention below is a silo; but it will be obvious to those skilled in the art how to apply the principles of the present invention to any type of bin.
Five principal methods are known for continuous measurement of the content of a bin such as a silo.
An electromechanical (yo-yo) level sensor consists essentially of a weight at one end of a reel of tape. The weight is allowed to descend in the silo to the depth at which the top surface of the content is situated. When the weight settles on top of the content, the tension in the tape slackens. The weight then is retracted to the top set point. The height of the content is inferred from the time required to retract the weight or from the measured tape length.
Mechanical devices such as yo-yo sensors are unreliable. They tend to get clogged by dust and to get stuck on obstacles such as pumps and rods inside the silos.
Ultrasonic level sensors work on the principle of ultrasonic sound wave transmission and reception. High frequency sound waves from a transmitter are reflected by the top surface of the content to a receiver. The height of the content is inferred from the round-trip travel time. Such sensors have limited range and work poorly in the presence of dust. In addition, such devices need to be custom-designed for different types of silo.
Radar level sensors work on the principle of electromagnetic wave transmission and reception. Electromagnetic waves from a transmitter are reflected by the top surface of the content to a receiver. The height of the content is inferred from the round-trip travel time. Such sensors are have a single point of measurement that is not suited for bulk solids.
Capacitance sensors measure the capacitance between two metallic rods or between a metallic rod and the ground. Because the silo content has a different dielectric constant than air, the capacitance changes according to the level of the top surface of the content between the two rods or between a rod and the ground. Such sensors tend to be inaccurate and are sensitive to humidity and to type of material stored in the silo.
All the prior art sensors discussed above are insensitive to the shape of the contents, and so are inaccurate in the presence of a common phenomenon called “coning” that occurs as bulk particulate solids are withdrawn via the base of a bin: an inverted conical hole, whose apex is directly above the point of withdrawal, tends to form in the bulk particulate solids. A similar phenomenon occurs as bulk particulate solids are added to a bin from the top: the solids tend to pile up in a cone whose apex is directly below the point of insertion of the solids. These sensors also work poorly in bins with complicated geometries and in the presence of obstacles.
A weight gauge measures the weight of a mobile silo and its content by measuring the tension in the rods that hold the silo. Installation of such gauges is complex, and they are suitable only for mobile silos with metallic legs.
One of the main issues in measuring the contents of a bin (bulk solids or liquids) is false echoes.
False echoes results from an illumination of obstacles within the bin. These obstacles can be foreign objects, irregularities on the bin walls, build up of content material and even corrugations). Content material can, for example, accumulate on an obstacle or on the bin wall.
When material level is bellow these obstacles—the level measurement device might lock on the obstacles and consider them as the actual material level.
The location of these obstacles and their sizes is different in every bin and therefore they cannot be eliminated prior to installation in a generic robust way for all vessels.
A known way to cope the false echoes problem is to wait till the bin is empty of contents (after level measurement device is installed) and then map (record) manually all the echoes the device detect that come from distances that are smaller than the vessel bottom. In more general sense the mapping can be done till a given known material level (that might be above the bottom).
There are few major problems with this method:                a. Manual mapping requires some technical understanding of the device operation (which makes the installation easy to use).        b. Manual mapping requires the knowledge of the contents level at a given time (which is actually why the device was installed in the first place).        c. At some cases the vessel will be emptied only a long time after installation which prolongs the completion of installation        d. Build ups of bulk solids create obstacles that grow over time and thus no manual mapping can eliminate them in advance.        
There is thus a widely recognized need for, and it would be highly advantageous to have, a method of measuring the content of a bin and especially detect obstacles and would overcome the disadvantages of presently known methods as described above.